The invention relates to a resonant converter comprising a control system, a control method for a resonant converter and a switched-mode power supply.
A switched converter converts a DC voltage on the input side into one or more DC voltages on the output side in that the input-side DC voltage is first chopped, i.e. converted into a switched AC voltage and with this switched AC voltage a resonant circuit comprising at least one capacitor is supplied with power, which capacitor comprises the primary side of a transformer. On the secondary side the transformer comprises one or various windings whose voltages are rectified for generating DC output voltages.
Known switched-mode power supplies comprise a power supply input circuit for connection to the mains and a switched converter. The switched-mode input circuit renders an intermediate-circuit DC voltage available with which the switched converter is supplied. The intermediate-circuit DC voltage is converted into one or more output DC voltages by the converter.
Many circuits for switched converters are known. This comprises in addition to resonant converters also circuits in which no resonant circuit is used. With converters of this type it is possible to manufacture inexpensive, small, lightweight power supply units/switched-mode power supplies, which can advantageously be used in consumer electronics appliances such as set top boxes, satellite receivers, television sets, computer monitors, video recorders and compact audio systems. In these applications there is often a need for converters that generate multiple output voltages on multiple converter outputs from one input DC voltage.
At least one of the output voltages is customarily controlled to a set value. In state-of-the-art converters which produce a plurality of output voltages and each of the output voltages is assigned a secondary winding of the transformer, various output voltages cannot be controlled independently of one another. In such circuits a control device is provided for only one of the output voltages. It is then assumed that the other voltagesxe2x80x94which are related to a controlled voltage as regards the number-of-windings ratioxe2x80x94are xe2x80x9ccontrolled alongxe2x80x9d with these voltages. However, this has considerable disadvantages at the individual outputs in case of strongly differing loads.
A known topology of a converter comprises the so-termed load resonant converter. In a known circuit for this a half bridge supplied with a DC voltage is used as an inverter, which half bridge feeds a series combination of a resonant capacitor and the primary side of a transformer. The resonant capacitor together with the leakage inductance of the transformer as well as further, also secondary-side inductances or capacitances, forms a resonant circuit. On the secondary side the load resonant converter includes one or more secondary windings. In this way a number of output DC voltages is supplied which, after being rectified, are customarily filtered by at least one capacitive filter.
To control the output voltage of such a resonant converter it is known to change the driving of the inverter. The switches of the inverter are then driven so that an AC voltage, in many cases a pulse-width modulated voltage, is generated that has predefined parameters (for example, frequency). By variation of the frequency of this voltage, the value of the output voltage can be controlled. The output voltage then rises the more the frequency comes nearer to the resonant frequency of the resonant circuit. LLC converters commonly operate in the hypercritical domain i.e. supply of the resonant circuit with a voltage whose frequency lies above the resonant frequencies. In this case the output voltage can be increased in that the frequency of the voltage is reduced. In known load resonant converters only one output voltage can be controlled directly. Further output voltages are coupled to the directly controlled output voltage via the number-of-windings ratio and are thus xe2x80x9ccontrolled alongxe2x80x9d.
The type of converter dominating in consumer electronics appliances is the flyback converter. This is a non-resonant converter. On the primary side generally only one switching element is necessary for the inverter. The flyback converter executes a one-way rectification on each of its outputs. One of the outputs is controlled directly.
If the flyback converter needs to have a second output voltage which must be controlled directly, it is known that a further converter known as a step-down converter or Buck converter is to be connected to one of the outputs of the flyback converter, which further converter is fed with the first output voltage of the flyback converter and produces the second output voltage with a separate control. Such a circuit comprising two converters is very costly, however.
Another extension of the flyback converter topology which renders two controlled output voltages available is the so-termed xe2x80x9cdouble forward-flybackxe2x80x9d converter. A respective topology is described, for example, in IEEE-PESC 1988, p. 142 xe2x80x9cA Complete Study of the Double Forward-Flyback Converterxe2x80x9d by J. Sebastian et al. As with the basic flyback topology, this is not a resonant circuit, but the primary-side AC voltage which is generated via a simple switch directly feeds the primary side of the transformer. On the secondary side there are two secondary units each formed by a secondary winding of the transformer and one one-way rectifier element (diode). The resulting secondary voltages are filtered capacitively by one secondary unit and inductively by the other secondary unit. In this way it is possible to control an (inductively filtered) output voltage via the duty cycle of the pulse-width modulated voltage and the other (capacitively filtered) output voltage via the frequency of the pulse-width modulated voltage. But this xe2x80x9chard switchingxe2x80x9d topology has considerable switching losses.
In modem consumer electronics appliances it is ever more necessary for two supply voltages can be controlled separately.
It is therefore an object of the invention to provide a resonant converter and a control method which can be realized cost effectively and nevertheless offer the possibility of control of a plurality of output voltages.
This object is achieved by a resonant converter as claimed in claim 1 and a control method as claimed in claim 13 and a switched-mode power supply as claimed in claim 14. Dependent claims relate to advantageous embodiments of the invention.
According to the invention a resonant topology is proposed i.e. a resonant circuit is fed by an inverter which resonant circuit comprises, for example, a series capacitance and the primary side of a transformer. Also further, secondary-side elements may be part of the resonant circuit. In such a resonant topology the output voltage can be controlled via the frequency of the primary-side AC voltage. By hypercritical operation there may be achieved with such a resonant converter that the resonant circuit at the source behaves as an inductive load, so that a switching without losses is possible (zero-voltage switching).
The separate control of different output voltages is made possible according to the invention in that two types of secondary units are provided, each formed by a secondary winding of the transformer and at least one rectifier element. A first secondary unit (a first type of secondary units, respectively) and a second secondary unit (a second type of secondary units, respectively) here have opposite orientations. The orientation is here to be understood as the direction of winding in conjunction with the wiring with the rectifier element. For example, two secondary units of opposite types may be distinguished in that with otherwise the same circuit the direction of winding on the common transformer core is opposite. It is also possible, when two secondary windings have the same direction of winding, that the secondary unit of the first and second type are distinguished by a respective converted wiring. Wiring is here understood to mean the connection of the rectifier element, which is preferably a one-way rectifier element, for example, a diode, incorporated in one branch.
The distinction in two oppositely oriented types of secondary units provides that the two secondary units behave differently depending on their excitation. When operated with an AC voltage, the secondary units of the first and second types are powered successively. In essence, during a positive voltage peak on the primary side of the transformer, a current flows through the secondary unit of the first type. During the negative voltage peak on the primary side of the transformer a current accordingly flows through the secondary unit of the second type. As will be explained in detail hereinbelow, it is possible to utilize this distinction in a purposeful manner while more or less power is supplied by a secondary unit of the first or second type through an accordingly purposeful excitation.
The secondary units may also be DC isolated. But it is also possible for two secondary units to have a common tapping point.
Secondary voltages arise on the secondary units as a result of rectification by the rectifier element. These secondary voltages can be directly used as an output voltage, customarily after (preferably capacitive) filtering. Such an output in which solely a secondary unit produces the output signal, is referred to here as the direct output. But there may also be provided that a xe2x80x9cstack outputxe2x80x9d is used whose output voltage drops via one or more secondary voltages of a secondary unit of the first type and one or more secondary voltages of secondary units of the second type which are connected in series, for example. Examples for such topologies will be explained hereinafter with reference to the Figures.
The control system according to the invention controls both the first and the second output voltage to a desired value by driving the inverter. The inverter produces a switched AC voltage, preferably a pulse-width modulated voltage of customarily constant amplitude. Preferably two control variables are used for the separate control of the two output voltages, which control variables predefine the waveform of the pulse-width modulated voltage. Various proposals provide for this, on the one hand, the predetermined switching frequency and duty cycle of the pulse-width modulated voltage and the predefined switch-on times for the positive and negative voltage pulse, on the other.
A modulator which drives the inverter based on predefined values of the control device is preferably used in that a pulse signal is predefined for driving the switch of the inverter. Especially with low powers a half bridge is preferred as an inverter for reasons of cost-efficiency, with which half-bridge voltage pulses are generated from an input DC voltage by alternately switching two switches. A full bridge may also be used in this respect.
A general model with which controls are combined for various topologies will be explained hereinbelow with reference to the Figures. Hereinafter several further embodiments of the invention relating to concrete controls will be discussed.
According to a first proposal, control error sizes are determined for two output voltages for the separate control of the two output voltages. This comprises the formation of the control difference and preferably also a scaling. Such a scaling is particularly advantageous when the two voltages differ considerably. Furthermore, a sum and a difference size is determined from the control errors, the sum size depending on the sum of the control errors and the difference size on the difference between the control errors. By xe2x80x9cdependingxe2x80x9d is meant here that further operations, for example, scaling or other operations can be applied to the individual errors or to the sum or difference, as appropriate. Sum and difference sizes may also be directly equal to the sum of the difference of the control errors. Finally, the sum size predefines the frequency and the difference size the duty cycle of the pulse-width modulated voltage, while preferably one-dimensional control units, for example, I, PI or PID controllers are used.
According to a second proposal two voltages are controlled separately in that first, as described above, control error magnitudes are determined for the two output voltages to be controlled. A default value for the duration of a first positive voltage pulse of the pulse-width modulated voltage is determined from the first control error sizexe2x80x94preferably by a one-dimensional controller, for example PID controllerxe2x80x94and from the second control error size a default value for the duration of a negative voltage pulse (when a full bridge is used) or the duration in which the value of the pulse-width modulated voltage equals zero (when a half bridge is used). This control is preferably combined with the default of a minimum and/or maximum duration for the respective voltage pulse, to lay down an operating range for the switching frequency. In a hypercritical area, that is to say, area with a switching frequency beyond the resonant frequency/frequencies of the resonant circuit, the allocation is to be selected such that by setting the duration of the positive voltage pulse, the secondary voltages of the secondary units of the second type (which are powered in essence during the negative voltage pulse!) are controlled and vice versa.
Control units for realizing said proposals may be designed in many different ways. Integrated or discrete analog or digital circuits may be considered then. The control units may also be completely implemented as a digital control algorithm running on a microprocessor. For a cost-effective solution is especially preferred a realization as an analog circuit comprising at least one error unit for the formation of an error size signal and at least one multivibrator for generating a switching signal to control the inverter.
A resonant converter according to the invention may produce two controllable output voltages which are separated from each other. If more than two output voltages are necessary for an application, they are divided into two groups and the voltages of each group can be controlled separately from the voltages of the other group. The formation of these groups is then made such that secondary units of the first type produce the output voltages of the first group, whereas secondary units of the second type produce the output voltages of the second group. Alternatively, the second group of output voltages can also comprise such voltages that are supplied by secondary units of both the first and the second type (stack output).
These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiment(s) described hereinafter.